This invention is concerned with an improvement to a second stage centrifugal separator circuit for the separation of hydrocarbons from degritted, diluted bitumen froth. More particularly, the invention relates to the combination of a plurality of disc-nozzle centrifugal separators with a single capacitance vessel into which the excess heavy phase outputs of the separators discharge. Hydrocarbons, which may be present in the heavy phase streams, are separated in the vessel and a reservoir or pool of hydrocarbons-free water is maintained to supply fluid to the separators if required.
The invention has application as part of the known hot water extraction process for recovering bitumen from bituminous sands. In this process, the sands are mixed with hot water, steam and a dispersant, such as sodium hydroxide, in a rotating tumbler to heat and dilute the tar sand and initially disperse the contained bitumen. The thick slurry which is produced is further diluted with hot flood water and is then introduced into a primary separation vessel, where the bitumen, contaminated with some solids and water, forms primary froth, which is recovered. A middlings stream, containing bitumen, is withdrawn from the mid-section of the primary separation vessel and is passed through a sub-aerated secondary separation cell. Bitumen, contaminated with solids and water, is recovered in the form of secondary froth from the cell. This secondary froth is settled to remove some contaminants and is then blended with the primary froth. The combined froth is heated and deaerated and diluted with naphtha to alter the specific gravity of the bitumen. This stream is then introduced into a two stage centrifugal separation operation. In this operation, the diluted froth is first degritted by passing it through one or more scroll-type centrifugal separators to remove coarse solids. The product, comprising bitumen, fine solids and water, is then conventionally treated in a circuit comprising one or more disc-nozzle centrifugal separators to separate the water and solids from the bitumen and produce a relatively clean bitumen product.
The present invention is concerned with the excess heavy phase discharges from the disc-nozzle centrifugal separators. These separators are typically of the type sold by DeLaval Company Limited under the model designation -SX320T. The degritted, diluted bitumen-containing froth feed stock to the separators typically comprises 75% by weight hydrocarbons, 4% solids and 21% water. It is fed centrally into the machine and passes through a distributor into a disc stack zone, which is whirling in conjunction with the separator bowl. Due to the action of centrifugal forces, the heavier water and solids components move from a cylindrical interface zone outwardly toward the bowl, where they form a heavy phase pool, from whence they are discharged through nozzles; the lighter hydrocarbons move inwardly from the interface zone through the disc stack to a centripetal pump means involving a light phase paring disc. This pump means forces the hydrocarbons out through a central discharge line. A second centripetal pump means, also involving a paring disc, is located at the base of the machine and communicates with the heavy phase pool. If the separator is fed froth which contains more water than is needed to satisfy the separator bowl nozzles, the excess heavy phase fluid is pumped out of the separator through a bottom outlet by the second centrifugal pump means. Under normal operating conditions, this heavy phase discharge is water; under upset conditions, however, the heavy phase discharge can comprise as much as 75% by weight hydrocarbons, 4% solids and 21% water. If the separator feed is low in water and the heavy phase pool is being depleted, water or heavy phase fluid is introduced through the heavy phase paring disc into the pool to maintain the interface zone within its normal operating region. It will be appreciated, therefore, that the position of the interface is affected by the fluid backpressure at the outlet of the second or heavy phase centripetal pump means.
It is conventional to connect an auxilliary vessel, containing a reservoir of heavy phase fluid under pressure, with the second centripetal pump means. This vessel accepts excess heavy phase fluid from the separator and supplies same to it when the heavy phase pool is being depleted.
Heretofore, it has been the practice to provide a separate auxilliary or capacitance vessel with each separator. However, this is expensive; therefore it would be advantageous to use a single capacitance vessel with a bank of separators. However, certain problems arise when this is considered. Firstly, the separators produce excess heavy phase streams at varying rates -- if the rate of discharge of one or more of the separators is low, here is a danger of plugging the capacitance vessel with deposited solids. Secondly, if the hydrocarbons -- heavy phase interface within an operating separator is shifted too far toward the outside of the bowl, as can occur when a slug of high hydrocarbons content feed enters the separator, a large volume of hydrocarbons can suddenly escape into the capacitance vessel. These hydrocarbons can then back up into another separator which is taking fluid from the vessel reservoir and the interface will be "lost" in that separator as well. That is, the second separator will begin producing hydrocarbons through its bottom outlet. In this manner, by a process termed `short-circuiting`, the entire bank of separators can swiftly be rendered inoperative.